Hardtails might be dead in many mountain bike circles, but not in racing. While full-suspension continues to make inroads, hardtails hold fast in many World Cup teams and local racers’ stables.

Fans of the breed have a new choice for 2015, as Cannondale offers up a fresh platform, the F-Si. It’s Cannondale’s first update to its hardtail design since 2011’s Flash/F29 and, as you’d expect, was developed with the company’s World Cup racing team, headlined by 2012 Olympic bronze medalist Marco Fontana and 2013 world championships silver medalist Manuel Fumic.

The F-Si isn’t a radical departure. It’s conceived as a refinement of existing designs, albeit significant in some areas. The clue is in the name: Cannondale has used the F designation for its hardtails for decades, while Si stands for system integration.

The F-Si uses Cannondale’s long-standing willingness to tinker with industry standards to address what Cannondale sees as some of the shortcomings of existing 29er hardtails: low-speed handling that turns floppy and the added flex of the larger wheels. Combined with the longer wheelbase required by the bigger hoops, that all makes 29ers tougher to control in tight quarters.

At the same time, the company wanted to preserve the renowned high-speed stability of the larger wheel size. While Scott has gone to the middle 27.5-inch wheel size, Cannondale says it feels the foreseeable future for hardtails is mostly 29-inch. So solving 29ers’ most pressing issues came down to messing with accepted geometry norms, said Peter Denk, Cannondale’s director of technology.

Updated 29er Geometry
Cannondale slackened the head angle on the new F-Si to 69.5 degrees and bumped the fork offset (distance from the steering axis to front hub) out to 55mm. That’s 1.5 degrees slacker than the F29 on the head angle and an extra centimeter of offset, or rake (many 29er hardtails have head angles in the 71-degree range and 45-47mm of offset depending on fork spec). This is one area where Cannondale can make changes others can’t; because they make their own suspension forks, Cannondale isn’t restricted to fork makers’ offset offerings.

The result of all that monkeying around is not immediately apparent: the F-Si’s 79mm trail figure is just 1mm shorter than the F29. But, says, Denk, how you get there is what’s important. “Head angle isn’t speed sensitive,” he points out, “but offset is.” With increased offset, “the faster you go, the higher the momentum in the front wheel, which increases the torque you need to turn the handlebar.” The intended result is a more stable, locked-in feeling when swooping down singletrack.

The added offset also is supposed to help stop the feeling of falling into turns on low-speed terrain, like uphill switchbacks. The longer offset gives the bike a longer front-center (distance from the bottom bracket to front axle) which, along with a slightly slacker seat angle and shorter chainstays, positions the rider a bit more over the rear wheel.

Cannondale shortened the chainstays by 1.5cm over its old design, to 42.9cm, using a neat trick: it shifted the entire drivetrain 6mm outboard with an asymmetric dropout, much like fat bikes have used for years.

While the rear hub and drivetrain have shifted 6mm, the rear wheel remains centered. (Joe Lindsey)

Moving the drivetrain out cleans up the rear tire/front derailleur clearance issues that often result from tight rear triangle geometry and requires longer stays. Our test bike had a SRAM XX1 drivetrain without a front derailleur, but the bike is drilled for a direct-mount derailleur and two of the models feature 2x setups. Thanks to a new crankset spider, the 6mm shift does not change the bottom bracket or distance between pedals.

To get the rim to line up with the center of the frame, it’s now positioned 6mm to the left over the hub, which means a more even spoke bracing angle between the left and right hub flanges. That makes for a more stable wheel since the spoke tension is more evenly balanced. Cannondale says that the setup will still clear tires up to 2.35 inches wide.

New Lefty Options
Unlike traditional forks, Cannondale’s Lefty fork design requires specific head-tube dimensions to attach properly. The F29 head tubes were 134mm tall across all sizes, relying on a single Lefty. The new Lefty 2.0 fork uses three size-specific clamp dimensions, which lower the front end considerably for smaller-size bikes. On a small and medium F-Si, it’s now 97mm. That’s a crucial change given that one of the biggest issues smaller riders have with 29ers is a too-tall handlebar due to fitting the frame around big wheels and suspension. Large and extra-large head tubes were also reduced to 110mm and 122mm. The new, lower cockpit better balances the rider’s weight, which means less front wheel pop on steep climbs.

Ride Impressions
We rode the new F-Si in Albstadt, Germany, at the recent UCI World Cup. Albstadt is located in the rolling hills of southern Germany, almost to Switzerland. The trails are narrow and steep, zig-zagging their way up and down with tight switchbacks.

The terrain isn’t overly technical, but the heavily forested area gets a fair bit of rain. The narrow trails are cut into steep hillsides, with short, sharp climbs and flowing, fast descents broken up by sections of densely packed switchbacks. The decomposed limestone surface alternates from bony rock chip to grippy hardpack to greasy mud, while last year’s fallen leaves hide roots that are slick with moisture. Chose one bad line, and the front wheel will ricochet right off the track. In short, it’s an ideal place to test an XC bike.

What immediately stands out about the F-Si is its efficiency. Drivetrain response is instant. Stand up and jump on the pedals and the bike rockets forward. The short stays and rearward weight bias keep the back wheel firmly planted. Denk says that the F-Si is essentially a mountain bike analogue to Cannondale’s SuperSix EVO road frame.

Ripping down tight, rollercoaster singletrack, the high-speed stability is evident. A root or rock can still bounce the bike off line, but you can point and shoot, carving through sinuous turns and punching through short rough sections. The stability remained when I slowed for drop-ins or sections of stairs.

I had less success with the uphill switchbacks. The shorter head tube means a lower handlebar, which helped keep the front wheel down, but the long front center, longer wheelbase, and slack head angle made the tighter 180s tough. If you already had the skills to ride them, you certainly could on the F-Si, but I don’t think it can transform anyone’s handling skills.

The redesigned Lefty Carbon XLR 2.0 suspension fork features a larger diameter leg for added stiffness and new, higher-flow oil ports. The sight of a single-leg design will always freak some people out, but I found the system worked flawlessly, soaking up the modest-sized rocks and bumps without noticeable binding or stiction and tracking straight, tight lines with precision.

Hardtails aren’t known for a smooth ride, but the F-Si was a pleasant surprise. Yes, it’s still a hardtail. And no, I didn’t consciously notice any real flex from the Save 2 seatpost, but I ended our first, two-hour ride feeling fresh.

Cannondale claims, as do most manufacturers at new-bike introductions, that its new product wins key head-to-head tests against competitors’ offerings on things like total system weight and stiffness-to-weight ratio; they cite independent lab testing from Zedler, which does performance testing for most manufacturers and some publications.

We don’t make a practice of repeating manufacturers’ claims; the truth quickly gets lost in a blizzard of competing claims and sometimes the differences are relatively small. What’s interesting is that Cannondale doesn’t make a big deal out of them.

To them, a cross-country race isn’t won or lost because a bike is a few grams lighter, or a bottom bracket a few percent stiffer. The F-Si is plenty light; claimed weight for a large frame is 960 grams, and a complete Team Carbon model, with SRAM XX1 and Enve carbon-fiber rims, comes in at just under 19 pounds without pedals. And I certainly won’t complain about its stiffness. But Cannondale says that, insofar as the machine helps determine race results, the key factors are things like traction and handling.

That’s a pretty refreshing viewpoint. Cannondale also makes no secret of the fact that they don’t use a high proportion of high-modulus carbon fiber, which is very stiff, but brittle. Most bikemakers don’t use lots of HM fiber, actually, but HM and UHM and other monikers abound in the arms race of who’s got the best black magic. (Curiously, Cannondale can’t help itself on this front; it lists the frame in its spec sheets as “Hi-Mod” carbon.)

Instead, Denk said, he tries to focus on durability, toughness, and safety. There is some HM fiber in strategic spots to add torsional stiffness, but Cannondale uses mostly higher-strength intermediate-modulus fiber and a resin originally developed for composite baseball bats, which makes for a very tough frame. “I want the frame to deform hugely before it cracks,” he said.

The F-Si isn’t for everyone. It’s not just a hardtail, but a race bike: a purpose-built machine that isn’t going to be your favorite bike to take on long trail rides. MSRP isn’t set yet, but the lowest-level bike in the line, the Carbon 2, features the same frame as the team version, a SRAM X01 drivetrain and Stan’s ZTR Arch wheels. It won’t be cheap, and the price only goes up from there. But it does show what’s possible when companies aren’t afraid to push the boundaries and create their own interfaces rather than rely on industry standards. The new bikes will be available for sale this August.

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